The quality of measurement of a mass spectrometer is related to the vacuum conditions in the mass analyzer. In general, any collision of an analyte ion with another particle inside the mass analyzer changes the trajectory of the analyte ion which in turn has a negative impact on the quality of the measurement. The rate of collisions depends upon the collision cross section σ of ions in the spectrometer, the pressure in the device, and the energy of the ions.
In “Application of the Orbitrap analyzer for Measurement of ion-neutral collision cross sections”, Proc. 49th ASMS Conf. on Mass Spec. and Allied Topics, Chicago, Ill., May 27-31, 2001, Hardman et. al. derive a relationship between the pressure within an ORBITRAP™ mass spectrometer, the collision cross section of ions and the resolving power.
Accordingly, it is considered important to monitor the pressure within the ion trap/analyzer of a mass spectrometer, in order to verify that any analysis will be, is, or has been carried out under acceptable conditions. The most common way to do this is to employ an ion gauge located close to the mass analyzer. The exact type of gauge employed depends upon the type of mass analyzer and the pressure range over which it is typically operated. For example, Time of Flight (TOF) mass analyzers are usually operated at higher pressures than analyzers employing axially harmonic trapping, such as ORBITRAP™ or Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometers. The longer the ion path, the better the pressure should be.
There are several drawbacks of existing methods of pressure monitoring in a mass analyzer. Firstly, UHV gauges tend to be expensive. Secondly, the gauge cannot be positioned within the analysis region. Although the gauge may be positioned close to it, still the pressure measured by the gauge is not the actual pressure where the ions travel so that the measured pressure can only permit an inference of the true conditions where the ions are travelling, are trapped, or are measured. Due to instrument geometry and field considerations, the gauge in FT/MS instruments such as Orbital or ICR is frequently placed away from the actual analysis device. The pressure reading of the ion gauge also depends upon gas type, and the composition of the gas is not known as it depends on several factors such as gas inlets or pumping characteristics of the attached vacuum pumps. The composition of the gas also changes the collision cross sections.
Against this background, it is an object of the present invention to provide an alternative and/or improved method of assessing the vacuum conditions in the ion trap of a mass spectrometer, particularly of the type which generates a transient detection signal.